In a typical boiling water nuclear reactor, a nuclear reactor core is submerged in water in an annular reactor pressure vessel for heating the water to generate steam which is used to power a steam turbine-generator for producing electrical power for example. In order to replace the water lost by the production of steam, one or more feedwater spargers are provided inside the pressure vessel and supplied with feedwater through feedwater nozzles extending through the pressure vessel to maintain a nominal level of water therein.
The water within the reactor pressure vessel is typically circulated therein by pumps for cooling the reactor core. In alternate designs, pumps are not used for water circulation, but natural circulation is used instead in one type of reactor called a simplified boiling water reactor (SBWR). However, to achieve a suitable driving head for recirculation flow, the vessel includes a relatively high chimney positioned above the reactor core and radially inwardly from the inner surface of the vessel to form an annular downcomer therebetween. The chimney includes suitable vertical partitions therein for maximizing the upward buoyancy effects of the heated steam and water mixture from the core, with relatively cool water flowing downwardly through the downcomer since it has a higher density. The feedwater is introduced into the vessel at the top of the downcomer and is relatively cold water which joins the downcomer flow in the natural recirculation within the pressure vessel.
The feedwater sparger may be in the form of a full 360.degree. ring disposed at the top of the downcomer or may be in the form of a plurality of arcuate feedwater spargers which collectively form the ring. In either embodiment, the spargers occupy the space at the top of the downcomer and thusly reduce access to the relatively narrow downcomer annulus, which is undesirable during a maintenance outage of the reactor. During such an outage, access is required in the region between the chimney and the vessel for inspection and/or maintenance thereof as well as for access to the various nozzles located in the downcomer region below the spargers. Accordingly, a removable feedwater sparger is desired for providing access to the downcomer during a maintenance outage of the reactor.